This invention relates to a vehicle transmission, for interposition between a prime mover of the vehicle and ground-engaging means of the vehicle and providing a plurality of gear ratios by which the ground-engaging means can be driven by the prime mover, and particularly but not exclusively to a vehicle transmission for material-handling vehicles. By “material handling vehicle”, we mean, for example, a tractor, loader type or similar vehicle for industrial and/or agricultural use, in which the ground engaging means typically comprises wheels, but could also include crawler tracks.
Vehicles such as tractors or loaders conventionally have utilised transmissions which provide a range of different gear ratios, for example four ratios. Such a transmission typically includes a plurality of clutches, one of which is conventionally known as the “forward” clutch, and one of which is conventionally known as a “reverse” clutch. The “forward” clutch is operable to engage a gear(s) within the transmission to effect rotation of the output shaft such that the vehicle moves forwards and the “reverse” clutch is operable to engage a further gear(s) within the transmission to effect rotation of the output shaft in an opposite direction. At least one other clutch is also provided which is operable to engage yet further gears within the transmission so as to provide different gear ratios.
In conventional syncromesh gearboxes, in order to change gears it is necessary to disengage the “forward” or “reverse” clutch, operate at least one other clutch to select a higher or lower gear, and then re-engage the forward or reverse clutch. This can result in a slow gear change which can result in a significant torque loss or even momentary complete loss of torque.
It is known to provide “powershift” gearboxes in vehicles such as performance cars on which, to overcome the problem of broken torque during gear shifting, gears are selected not by moving teeth but by the use of multi-plate, normally wet, clutches. Multi-ratio power shifting gearboxes, with a multitude of hydraulically operated multi-plate clutches, typically, have constantly meshing gears continually being rotated in both driven and idle states. The idling gears in turn drive one half of the multi-plate clutch assemblies which creates both heat and inertia drag due to oil churning etc. These inertia/drag loads are significant and can create power losses, leading to higher fuel consumption and/or longer journey times in high speed machines.
Twin clutch gearboxes attempt to provide the unbroken torque available from powershift gearboxes without the loss of efficiency. Successive gears are provided on alternate lay shafts. Conventionally, such twin clutch/alternate lay shaft gearboxes have the clutches located in line with the alternate lay shafts which makes the gearboxes relatively long.
It is an object of the present invention to provide a new or improved vehicle transmission.